1. Field of the Invention
The present invention relates to an air bag cover made of synthetic resin which can be used to cover a folded air bag of an air bag device in an instrument panel, a door trim, a seat back, or a steering wheel of an automobile and manufacturing method thereof.
2. Description of the Prior Art
Conventionally, an air bag cover comprises a synthetic resin core member and a decorative sheet disposed on the front surface of the core member (see Japanese Utility Model Publication No. Sho. 63-22262 for example).
The conventional air bag cover includes a breakable portion which, when the air bag is expanded, can be easily broken to allow the expanding air bag to project outwardly. This breakable portion is produced by forming a recessed portion in the air bag cover by means of high frequency, heat, or supersonic waves (see Japanese Patent Publications Nos. Hei. 2-248238, 4-151345, 6-218811 for example).
When the recessed portion is formed on the side of the air bag cover visible to a passenger of an automobile, the design quality of the air bag cover is lowered. For this reason, the recessed portion is conventionally formed on a back side of the air bag cover. Yet, in order to allow the air bag cover to break smoothly, the decorative sheet must also be provided with a recessed portion.
Conventionally, the decorative sheet is formed by superimposing three layers one on top of another into a united body, including an outer layer made of soft vinyl chloride or the like, a barrier layer made of polypropylene used to protect the outer layer from the heat and pressure associated with forming the core member, and a foaming layer made of foaming polypropylene, urethane or the like interposed between the outer layer and barrier layer. To allow the air bag cover to break smoothly, the conventional decorative sheet provides a recessed portion not only in the barrier layer but also in the back side of the outer layer.
However, the thickness of the outer layer is small, on the order of 1 mm, and, therefore, it is difficult to control the thickness of the outer layer when the recessed portion is formed. The thickness is even more difficult to control when the air bag cover is formed to have a curved surface and the breakable portion is formed in the curved surface.
Moreover, because the outer layer is made of soft vinyl chloride, the outer layer can become brittle at low temperatures, raising a fear that small fragments of the outer layer will be projected when the air bag deploys. This problem makes it necessary to strongly bond the outer layer to the foaming layer by adhesive.